1. FIELD OF THE INVENTION
This invention relates to a recording sheet. More specifically, the invention relates to a recording sheet comprising a coating of microcapsules having a volume average particle diameter and a number average wall thickness in a specified relationship, and a melt-extruded protective layer thereon.
2. DESCRIPTION OF THE PRIOR ART
A pressure-sensitive recording sheet is known as a typical example of a recording sheet comprising a support and coatings of a color former and a color developer. A pressure-sensitive recording sheet is composed of a support and coatings of a color former and a color developer. When microcapsules containing the color former are ruptured by pressures such as writing pressures, the color former chemically reacts with the color developer to form a recorded image.
Pressure-sensitive recording sheets of this kind have various defects since the coated layer on the support makes direct contact with the ambient atmosphere. For example, when they are stored in a place having a high humidity, or when water or oil adheres to them, a color reaction takes place to cause "color fog", and the characteristics of the product are impaired. Furthermore, they are susceptible to soiling by ink or dirt, etc., during use or storage, and are difficult to handle in storage. Moreover, the images recorded discolor with time, and this markedly impairs the commercial value of recording sheets.
In an attempt to eliminate such a defect, research and development work has been extensively done heretofore in regard to materials for the color former, the microcapsules, and the color developer. But so far a recording sheet has not yet been developed which has satisfactory resistance to moisture, water, oils, stain, light, and to abrasion.
A recording sheet comprising a support, a coating (recording layer) of a color former and a color developer, an adhesive layer and a transparent film, in this order, has also been known. This recording sheet has eliminated the aforementioned disadvantage, but still has the following disadvantages. Application of pressures is required in order to bond the recording layer and the transparent film. This results in the formation of "color fog" in the recording layer. Furthermore, this recording sheet has the disadvantage that the adhesive layer must be provided between the recording layer and the transparent film using a solvent. Since the adhesive layer during use contains the solvent remaining therein, difficulties such as a formation of "color fog" in the recording layer, a degradation of the working environment, or a restriction of the speed of producing the recording sheets, cannot be avoided.
It has been suggested on the other hand to measure planar or linear pressures using such recording sheets. But this method has the following disadvantages.
1. A linear relationship is not observed between the pressure applied and the density of a color formed.
2. "Color fog" ascribable to staining of the site of the pressure measurement tends to occur.
3. The density of color has poor stability with time. In other words, the accuracy of measurement is poor.